How do the roots of the crops we eat respond to atmospheric CO2 enrichment? Unless the roots are the edible portion of the plant, we probably give the question little thought: "out of sight, out of mind," as the saying goes. But it is the roots that acquire the water and nutrients that sustain the plant; and so the question is of more than academic interest, providing a significant impetus for scientists to look beneath the surface of the subject and the ground. Hence, we here review what a few of them have found.
Hodge and Millard (1998) grew narrowleaf plantain (Plantago lanceolata) seedlings for a period of six weeks in controlled environment growth rooms maintained at atmospheric CO2 concentrations of either 400 or 800 ppm. By the end of this period, the plants in the 800-ppm air exhibited increases in shoot and root dry matter production that were 159% and 180% greater, respectively, than the corresponding dry matter increases experienced by the plants growing in 400-ppm air, while the amount of plant carbon recovered from the potting medium (sand) was 3.2 times greater in the elevated-CO2 treatment. Thus, these investigators found that the belowground growth stimulation provided by atmospheric CO2 enrichment was greater than that experienced aboveground.
Wechsung et al. (1999) grew spring wheat (Triticum aestivum) in rows in a FACE study employing atmospheric CO2 concentrations of 370 and 550 ppm and irrigation treatments that periodically replaced either 50% or 100% of prior potential evapotranspiration in an effort to determine the effects of elevated CO2 and water stress on root growth. They found that elevated CO2 increased in-row root dry weight by an average of 22% during the growing season under both the wet and dry irrigation regimes. In addition, during the vegetative growth phase, atmospheric CO2 enrichment increased inter-row root dry weight by 70%, indicating that plants grown in elevated CO2 developed greater lateral root systems than plants grown at ambient CO2.
During the reproductive growth phase, elevated CO2 stimulated the branching of lateral roots into inter-row areas, but only when water was limiting to growth. In addition, the CO2-enriched plants tended to display greater root dry weights at a given depth than did ambiently-grown plants. This study thus suggests that as the CO2 content of the air continues to rise, spring wheat plants will likely develop larger and more extensively branching root systems that may help them to better cope with periods of reduced soil moisture availability.
In a comprehensive review of all prior FACE experiments conducted on agricultural crops, Kimball et al. (2002) determined that for a 300-ppm increase in atmospheric CO2 concentration, the root biomass of wheat, ryegrass and rice experienced an average increase of 70% at ample water and nitrogen, 58% at low nitrogen and 34% at low water, while clover experienced a 38% increase at ample water and nitrogen, plus a 32% increase at low nitrogen. Outdoing all of the other crops was cotton, which exhibited a 96% increase in root biomass at ample water and nitrogen.
In a radically different type of study, Zhao et al. (2000) germinated pea (Pisum sativum) seeds and exposed the young plants to various atmospheric CO2 concentrations in controlled environment chambers to determine if elevated CO2 impacts root border cells, which are major contributors of root exudates in this and most other agronomic plants. They found that elevated CO2 did indeed increase the production of root border cells in pea seedlings. In fact, in going from ambient air to air enriched to 3,000 and 6,000 ppm CO2, border-cell numbers increased by over 50% and 100%, respectively. Hence, as the CO2 content of the air continues to rise, peas (and possibly many other crop plants) will likely produce greater numbers of root border cells, which should increase the amounts of root exudations occurring in their rhizospheres, which further suggests that associated soil microbial and fungal activities will be stimulated as a result of the increases in plant-derived carbon inputs that these organisms require to meet their energy needs. Last of all, this chain of events should make the soil environment even more favorable for plant growth and development in a high-CO2 world of the future.
In concluding this brief review, we revisit the study of van Ginkel et al. (1996), who grew perennial ryegrass (Lolium perenne) plants from seed in two growth chambers for 71 days under continuous 14CO2-labeling of the atmosphere at CO2 concentrations of 350 and 700 ppm at two different soil nitrogen levels. At the conclusion of this part of the experiment, the plants were harvested and their roots dried, pulverized and mixed with soil in a number of one-liter pots that were placed within two wind tunnels in an open field, one of which had ambient air of 361 ppm CO2 flowing through it, and one of which had air of 706 ppm CO2 flowing through it. Several of the containers were then seeded with more Lolium perenne, others were similarly seeded the following year, and still others were kept bare for two years. Then, at the ends of the first and second years, the different degrees of decomposition of the original plant roots were assessed.
It was determined, first of all, that shoot and root growth were enhanced by 13 and 92%, respectively, by the extra CO2 in the initial 71-day portion of the experiment, once again demonstrating the significantly greater benefits that are often conferred upon plant roots by atmospheric CO2 enrichment. Secondly, it was found that the decomposition of the high-CO2-grown roots in the high-CO2 wind tunnel was 19% lower than that of the low-CO2-grown roots in the low-CO2 wind tunnel at the end of the first year, and that it was 14% lower at the end of the second year in the low-nitrogen-grown plants but equivalent in the high-nitrogen-grown plants. It was also determined that the presence of living roots reduced the decomposition rate of dead roots below the dead-root-only decomposition rate observed in the bare soil treatment.
Based on these findings, van Ginkel et al. conclude that "the combination of higher root yields at elevated CO2 combined with a decrease in root decomposition will lead to a longer residence time of C in the soil and probably to a higher C storage," providing still further evidence that the contrary claims of Hungate et al. (2003) are considerably out of touch with reality [See our Editorial of 10 Dec 2003 and the Wetlands Summary in our Subject Index].
References
Hodge, A. and Millard, P. 1998. Effect of elevated CO2 on carbon partitioning and exudate release from Plantago lanceolata seedlings. Physiologia Plantarum 103: 280-286.
Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.
Kimball, B.A., Kobayashi, K. and Bindi, M. 2002. Responses of agricultural crops to free-air CO2 enrichment. Advances in Agronomy 77: 293-368.
Van Ginkel, J.H., Gorissen, A. and van Veen, J.A. 1996. Long-term decomposition of grass roots as affected by elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 1122-1128.
Wechsung, G., Wechsung, F., Wall, G.W., Adamsen, F.J., Kimball, B.A., Pinter Jr., P.J., LaMorte, R.L., Garcia, R.L. and Kartschall, T. 1999. The effects of free-air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth. Global Change Biology 5: 519-529.
Zhao, X., Misaghi, I.J. and Hawes, M.C. 2000. Stimulation of border cell production in response to increased carbon dioxide levels. Plant Physiology 122: 181-188.